Field of the Disclosure
This disclosure relates to multilayer films and authentication labels.
Description of the Related Art
Holographic images may be used for identification and authentication of products and may be applied directly to the products or the product packaging. The most common hologram used is a surface-relief hologram, formed by a patterning and stamping process in which a relief grating is formed on the surface of a recording medium, such as a photoresist material. In a surface-relief hologram, interference planes are predominantly perpendicular to the recording surface and the surface-relief grating of the recording medium provides for the diffraction of incident light, enabling the formation of the observed holographic image. As basic holographic technology becomes more widely available, however, such a simple hologram may be more easily counterfeited and applied to non-authentic products. Therefore, a surface-relief hologram has limited value as an authentication and/or security device.
Another type of hologram is a volume-phase hologram, in which a recording medium is a photosensitive material, such as a silver halide emulsion, a dichromated gelatin or a photopolymer. In a volume-phase hologram, the interference planes are not limited to just being perpendicular to the recording surface (as in surface relief holograms), but can be at a variety of angles with respect to the recording surface. Photo-imaging creates interference fringes throughout the depth of the recording medium that diffract light and create the observed holographic image. Unlike a surface-relief hologram, which does not reflect light, but only diffracts light in transmission, a volume-phase hologram can do both. Volume-phase holograms are more difficult and costly to produce than surface-relief holograms, but enable the production of a more detailed image that has greater three-dimensional character, complete with parallax, and is far more challenging to counterfeit.
A volume-phase hologram is often produced as an authentication label in the form of a multilayer film that is subsequently applied to a product using an adhesive layer. In some cases, counterfeiters will attempt to remove an authentication label from a genuine product or product packaging and reapply it to a non-authentic good. Often this is done by using heat (e.g., a heat gun) to soften the adhesive layer, and then carefully scraping the label off.
It would be desirable to produce an authentication label that cannot be readily removed from one substrate and reapplied to another. For instance, a label that is irreparably damaged upon the application of heat might discourage attempts to remove labels by softening the adhesive. Alternatively, a label that becomes highly frangible after the application of heat might prevent successful removal of the label.